Brain Amoebas. Organ Transplants. Brrr.

Nightmare scenario: New disease arises. It’s rare enough that there aren’t many known patients, and so it isn’t well-studied — and because it isn’t well-studied, there aren’t agreed-upon treatments or accurate tests for it, and there isn’t a good understanding of situations where the infected might pose an unusual risk to others.

Situations, for instance, like organ transplants.

The CDC’s weekly bulletin today describes that nightmare scenario come true. Last year, four people received the kidneys, heart and liver of a 4-year-old boy who died in Mississippi of encephalitis that was assumed to be a rare reaction to flu infection. Weeks after the transplant, the two kidney recipients developed neurological symptoms — spasms, seizures, visual disturbances — and were hauled back to hospitals for evaluation. MRIs showed ring-shaped lesions in both their brains. That sent investigators back to re-examine the boy’s death — and revealed that while he did have encephalitis, it wasn’t because of flu.

It was because he was infected with a newly recognized pathogen, Balamuthia mandrillaris, a species of amoeba. It had passed to the four recipients via his organs, and grew in them with an assist from the immune-suppressing drugs they were taking to prevent rejection.

There have been only about 200 human cases of Balamuthia in the world since the infection was first recognized in 1990 (it got its name from its first-ever sighting the year before, in a sick mandrill at the San Diego Wild Animal Park). Treatment is prolonged and complex, an IV cocktail of multiple drugs that can drag on for months and optimally includes a drug, miltefosine, that isn’t even available in the United States except under an emergency plea to the Food and Drug Administration. When the amoeba develops in the brain, treatment is often unsuccessful anyway: Most cases of Balamuthia encephalitis are fatal.

And so it was in this cluster. One kidney recipient, a 31-year-old woman, developed her first neurological symptoms 20 days post-transplant and died on day 75. The other, a 27-year-old man, survived with “residual right arm paralysis, bilateral leg weakness, and intermittent vision loss,” the CDC says. The heart and liver recipients, both toddlers, mysteriously did better: They had not yet developed symptoms when the kidney recipients were diagnosed, and were hospitalized for evaluation and given a multi-drug cocktail in time.

Could this have been detected earlier? Probably not: It sounds simply random, rare and sad. The boy who was the donor was from Kentucky but was living with relatives in Mississippi when he first got sick in October 2009. He was diagnosed with the flu, got antivirals, got better, and then a week later developed headaches and seizures; was hospitalized for three days, diagnosed with acute disseminated encephalomyelitis, an immune reaction that can follow on flu, and was discharged. Four days later, his seizures recurred, and he died a week after that, on Nov. 18. In the course of his treatment, he’d been checked for viral, bacterial and fungal infections, and all had come up negative — so the family and hospital let his organs go for transplant. The recipients got them two days after that.

It wasn’t until the kidney recipients fell ill that the boy’s tissue, preserved after donation, was checked by CDC investigators, who found something that wasn’t viral, or bacterial, or fungal. They found the Balamuthia amoebas, distributed throughout his brain.

Balamuthia lives in soil around the world, but no one is confident yet of how people become infected; the predominance of encephalitis cases, and some facial infections in milder cases, suggest the route may be as simple as inhaling dust that contains amoebic cysts. It can only be diagnosed by brain biopsy, usually at autopsy, and so there’s some concern that it may be misdiagnosed and be more common than we know. A few cases in California have been analyzed, because California happens to have an encephalitis registry.

But it’s a funny thing about new, rare diseases: Once you know what to look for, you start seeing it where you wouldn’t have detected it before. And so it is with Balamuthia — while the CDC staff were writing up this cluster, another report came in, of another transplant cluster that happened just a few weeks ago in Arizona. In this case, the donor was a 27-year-old landscaper who died July 21 of what people thought was a stroke. Arizona residents, both men, got his liver, and one kidney and his pancreas; illustrating how far organ-donation networks can reach, his heart went to a man in California, and the other kidney to a man in Utah.

The liver recipient died Aug. 17. The kidney-pancreas recipient died Aug. 30. Their illnesses alerted the transplant network, and the other two donors were put on a multi-drug cocktail. So far, they have survived.

Because I spend so much time thinking about scary diseases, people often ask me if there’s anything that gets to me. I wash my hands a lot. I follow whatever precautions are being taken by scientists I hang around with. Most of the time, in most of my life, I don’t feel like I’m much at risk.

But amoebas, growing in your brain, because you happened to inhale the wrong speck of dust? That squicks me. A lot. Brrr.